A common technique for mounting a cutting insert relative to a toolholder in a cutting tool is to form a pocket having a bottom abutment surface and two side abutment surfaces in the toolholder. A cutting insert having a bottom supporting surface and a plurality of side supporting surfaces is then clamped in the pocket so that the bottom supporting surface contacts the bottom abutment surface of the pocket and the side supporting surfaces contact the side abutment surfaces of the pocket. In this way, three points of contact are provided between the insert and the pocket, providing for necessary stability of the insert. Inserts and toolholders are typically not designed to have more than three points of contact between the insert and the toolholder. This is because the level of precision necessary to provide more than three points of contact is quite difficult to obtain and typically results in a situation sometimes referred to as “overdetermination,” whereby at least one of the intended points of contact is not in a good contacting relation and can cause the insert to be improperly located or permit undesired movement of the insert.
One technique for clamping an insert relative to the pocket of the toolholder is to provide a through hole in the insert with an inverted truncated conical shape at a top end of through hole, and a screw with a head having a generally matching conical shape on a bottom side of the head. The conical shapes of the head and the through hole contact as the screw is screwed into a threaded hole in the bottom abutment surface and the insert is thereby clamped relative to the pocket. Often, the location of the threaded hole is located so that, as the conical clamping surfaces come into contact when then screw is tightened relative to the toolholder and the insert, the side supporting surfaces of the insert are urged toward the side abutment surfaces of the pocket.
A disadvantage of this arrangement is that inserts tend to have slight variations in size. This may occur in a cemented carbide insert, for example, as the result of changes that occur in the shape of a pressed insert during sintering of the insert. The position of a cutting edge of the insert relative to the toolholder depends upon the distance between the points on the insert supporting surfaces that contact the insert abutment surfaces and the cutting edge. Variations in size from one insert to another can result in damage to a workpiece or poor quality finishes.
One solution to this problem has been to provide a “bottom locking” arrangement, wherein a projection is provided on the bottom abutment surface of pocket and is received in the through-hole of the insert. Contact between the projection and the interior surface of the through-hole can replace contact between side abutment surfaces of the pocket and side supporting surfaces of the insert. Because the distance between the point(s) at which the projection and the interior surface of the through-hole contact and the working cutting edge is less than the distance between the points at which side abutment surfaces contact side supporting surfaces and the cutting edge, the effect of variations in insert size is less pronounced.
Disadvantages with such bottom locking arrangements include that the projection is typically close to the bottom surface of the insert and, thus, there tends to be poor support for the insert during cutting operations. Also, the projections typically include sloped surfaces and the insert may tend to ride up the slopes when cutting forces are exerted on the insert.
Disadvantages to both bottom locking arrangements and arrangements wherein side abutment surfaces and side supporting surfaces contact include that, if the projection in a bottom locking arrangement or any abutment surface of pocket is damaged, it may not be possible to properly seat an insert against the surface. Repairs to the toolholder, if possible at all, can be complex and expensive.
It is desirable to provide a cutting tool that provides for accurate mounting of inserts. It is also desirable to provide a cutting tool that facilitates simple repair of damaged abutment surfaces on a toolholder.